In general, construction equipment includes a hydraulic system, and the hydraulic system receives power from an engine. The hydraulic system includes a hydraulic pump, a main control valve, an actuator, an operating unit, and the like.
FIG. 1 is a hydraulic circuit diagram illustrating a hydraulic system for construction equipment in the related art, and the hydraulic system for construction equipment includes a hydraulic pump 1, an actuator 2 driven by working oil discharged from the hydraulic pump 1, a spool 3 configuring a main control valve (not illustrated) provided between the hydraulic pump 1 and the actuator, an open center flow path 4 bypassing, that is, bleeding off, working oil discharged from the hydraulic pump 1 when the spool 3 is in a neutral state, a flow rate controller 5 receiving a negative flow control (NFC) pressure Pn detected by the open center flow path 4 and controlling a swash plate angle of the hydraulic pump 1 in order to adjust a flow rate of the hydraulic pump 1, and the like.
Particularly, when a driver operates an operating unit, such as a joystick, in order to drive the actuator 2, the spool 3 moves, so that the open center flow path 4 is decreased. Accordingly, a swash plate angle is adjusted so that the NFC pressure Pn is decreased, and a flow rate of the hydraulic pump 1 is increased. That is, the hydraulic system for construction equipment is controlled so that an input signal Pn of the hydraulic pump 1 is inversely proportional to an output signal (flow rate) of the hydraulic pump 1.
According to the hydraulic system for construction equipment, there is a problem in that working oil bypasses the open center flow path 4 when the hydraulic system for construction equipment stands by, so that a flow rate is lost, and pressure is lost according to a design of the spool 3, thereby degrading efficiency.
In the meantime, the hydraulic pump in the hydraulic system for construction equipment known in the related art includes a first pump and a second pump, which are flow rate control types, and an auxiliary pump. The first pump and the second pump provide working oil to the actuator performing an operation, and the auxiliary pump provides pilot working oil to an additional hydraulic device or a pressure receiving portion of the spool of a valve unit.
A plurality of valve units for distributing working oil to each actuator is provided inside the main control valve. Spools are provided in the valve units, respectively, and the valve unit is opened/closed according to a movement of the spool to control a flow direction of working oil to be a forward direction or a reverse direction. A movement displacement of the spool may be varied by the pilot working oil.
Spools of operating units, which the first pump and second pump take charge in, are determined, for example, the first pump may take in charge of a spool for a first speed of an arm, a spool for a second speed of a boom, a swing spool, an option spool, and a right travelling spool, and the second pump may take in charge of a spool for a second speed of the arm, a spool for a first speed of the boom, a bucket spool, and a left travelling spool.
The various spools may be complexly operated in order to perform an operation desired by an operator. For example, when excavating and loading operations are performed, soil is drawn up by operations of going down a boom, crowding an arm, and crowding a bucket, a boom goes up and an upper body swings, and then the soil is moved and drawn out by operations of dumping the arm and dumping the bucket.
Each actuator of the operating unit performs a series of operations, and a relatively small load is applied to the swing of the upper body, compared to a load applied to the boom up and the arm crowd.
The hydraulic system for construction equipment known in the related art equally distributes power of an engine to the first pump and the second pump. That is, when it is assumed that power of the engine is 100%, 50% of the power of the engine is distributed to the first pump and the second pump each, so that flow rates of the pumps are controlled.
As described above, a load is differently applied to a specific operation of a specific actuator among the various actuators. That is, a heavy load may be applied to the first pump or a light load may be applied to the second pump. In this case, it is recognized that the second pump relatively has a pump power margin.
In the hydraulic system for construction equipment known in the related art, the flow rate of the first pump, to which the heavy load is applied as described above, is controlled so that power of the first pump is increased, and the flow rate of the second pump, to which the light load is applied, is controlled so that power of the second pump is decreased.
The aforementioned control of the pump will be additionally described. The first pump and the second pump detect pump pressures thereof, and a swash plate angle of a corresponding pump is adjusted according to a size of a pump pressure of a counterpart pump. For example, when the pump pressure of the counterpart pump is high, the swash plate angle of the corresponding pump is controlled so that a swept volume of the corresponding pump is decreased, and when a pump pressure of the corresponding pump is high, a swash plate angle of the counterpart pump is controlled so that a swept volume of the counterpart pump is increased. Here, the swept volume (cc/rev) means a flow quantity discharged per unit revolution of the pump.
The control of the hydraulic system known in the related art has the problems below.
In order for the first pump and the second pump to serve as a corresponding pump controlling a counterpart pump pressure, working oil passes hydraulic lines and various valves, and in this process, pressure of the working oil is lost. Further, the pump power having a margin means that some of the power generated by the engine is not used and is wasted.
In the meantime, the engine combusts fuel to generate power, so that as described above, fuel is wasted by the amount of non-used power of the engine.
On the other hand, as described above, the first pump and the second pump according to the hydraulic system known in the related art limit horsepower with an average of the pressures, so that that is a problem in that the first pump and the second pump inevitably use horsepower control, in which a discharged flow rate is not considered, and it is impossible to use maximum horsepower generable by the pump in a specific operation form.
Further, it is set that engine horsepower is allocated to the first pump and the second pump according to the hydraulic system for construction equipment known in the related art at the same ratio, so that there is a problem in that it is impossible to differently set a distribution ratio of the engine horsepower even though a load applied for each operation mode or a load mode is different.